Low Warpage Flip-Chip Under-Fill Curing

Copper heat spreaders are often used in flip chip in package construction. While providing high thermal conductivity, Cu has a significantly higher coefficient of thermal expansion than Si. In this work, two heat spreader attachment materials, indium for high power and polymeric adhesive for medium power applications, have been investigated. For In solder based attach, the Cu heat spreader was metallized with Ni/Au. Two thin film metallizations, Ti/Ni/Au and Ti/Au, have been studied for the Si backside. A nearly void free heat spreader attach has been achieved with vacuum soldering. For Ti/Ni/Au backside metallized Si die, there was no significant shear strength change after 1000 hours aging at 120degC and there was no significant shear or pull strength variation after five lead free re flow cycles. The shear and pull failure mode was within the indium layer. For Ti/Au die backside metallization, the initial die pull strength and failure mode were a function of Au thickness. With 3000 A of Au, there is no significant variation for shear and pull strength after 600 hours aging at 120degC or after five lead free solder reflow cycles. Failure was in the indium layer. For both types of die metallization, 24 mm times 24 mm Cu heat spreaders assembled on 22 mm times 22 mm Si die, exhibited no delamination after two lead free solder reflow cycles followed by 500 air to air thermal shock cycles (-40degC to 85degC). At 1000 cycles, slight delamination was found at the edges of the assembly for both die metallurgies. For adhesive based flat heat spreader attachment, a thermally conductive adhesive was used as the thermal interface and a non-thermally conductive adhesive was applied at the substrate corners to provide mechanical reinforcement of the heat spreader. After pre-conditioning then aging at 100degC for 500 hours followed by 500 air-to-air thermal shock cycles (0degC to 100degC), no delamination was observed and there was no significant degradation in pull strength.

Polyimide films are commonly used on wafers as passivation layers, stress buffer layers, dry etch masks, structural layers, and re-distribution layers for chip scale packaging and wafer level packaging. These films are cured in convection or diffusion ovens at high enough temperatures (350-400/spl deg/C) to assure adequate mechanical and electrical properties. These high temperatures can change the electrical properties of the devices. If the cure temperature of these films were lowered there could be a reduction in the film(and wafer) stress as well as lower thermal budget for the devices. However, curing polyimide films at lower temperatures than 250/spl deg/C has not been found to provide the mechanical, chemical, and dielectric properties required for the final device operation parameters or mechanical and dielectric protection. This paper describes the use of variable frequency microwave (VFM) for the curing of existing polyimides at more than 50-150/spl deg/C lower than the standard convection cure temperatures while maintaining the necessary final mechanical and chemical film properties. These films can be etched, patterned, metalized and further processes to make multi-layer structures. Further, we report on the continuing development of optimized polyimide materials suitable for curing at temperatures less than 200/spl deg/C with VFM. Experiments to determine the relationship between the molecular structure of the polyimides and the efficiency of microwave susceptibility are also described.

Assembly of high performance photonic components continues to push the state if of the art for precision assembly. “Active Alignment” - a technique where measurements of part function are used to guide actuator position – is often required to achieve acceptable component alignment. These active alignment systems require rapid and reliable bonding of the part components as they are held in position. Tack cure of high performance adhesives is one of the methods used for such in-situ bonding. Adhesive bonding continues to be attractive given the continued rapid evolution of part structures and materials. However, concerns over machine throughput and thermal distortion often lead to compromises for adhesive selection and cure. In-situ cure is generally limited to a tack cure (partial cure) in order to guard throughput of the active alignment workcell. As described herein, microwave energy can be used to couple heat energy directly to the adhesive material and package interior. Specifically, Variable Frequency Microwave (VFM) can offer the speed and control of cure required to enable a cost effective, full in-situ cure. This paper presents information about this novel curing technology.

Product recovery strategy requires a thoughtful consideration of environmental implications of operational processes, undergone by a manufactured product in its entire product lifecycle, from stages of material processing, manufacturing, assembly, transportation, product use, product post-use and end-of-life. At the returns stream from product use stage, those parts and/or component assemblies from a used product have several disposition alternatives for recovery, such as direct reuse, remanufacture, recycle or disposal. Due to such complexity of the manufacturing processes in recovery, current decision methodologies focus on the performance measures of cost, time, waste and quality separately. In this article, an integrated decision model for used product returns stream is developed to measure the recovery of utilisation value in the aspects of cost, waste, time, and quality collectively. In addition, we proposed a model-driven decision support system (DSS) that may be useful for manufacturers in making recovery disposition alternatives. A case application was demonstrated with the use of model-driven DSS to measure recovery utilisation value for the used product disposition alternatives. Finally, the future work and contributions of this study are discussed.

How the crystallography and nanoscale chemistry of the metal/oxide interface develops during the aqueous oxidation of zirconium cladding alloys

Lead-free solder reflow process has presented challenges to no-flow underfill material and assembly. The currently available no-flow underfill materials are mainly designed for eutectic Sn-Pb solders. This paper presents the assembly of lead-free bumped flip-chip with developed no-flow underfill materials. Epoxy resin/HMPA/metal AcAc/Flux G system is developed as no-flow underfills for Sn/Ag/Cu alloy bumped flip-chips. The solder wetting test is conducted to demonstrate the fluxing capability of the underfills for lead-free solders. A 100% solder joint yield has been achieved using Sn/Ag/Cu bumped flip-chips in a no-flow process. A scanning acoustic microscope is used to observe the underfill voiding. The out-gassing of HMPA at high curing temperatures causes severe voiding inside the package. A differential scanning calorimeter (DSC) used to study the curing degree of the underfill after reflow with or without post-cure. The post-curing profiles indicate that the out-gassing of HMPA would destroy the stoichiometric balance between the epoxy and hardener, and result in a need for high temperature post-cure. The material properties of the underfills are characterized and the influence of underfill out-gassing on the assembly and material properties is investigated. The impact of lead-free reflow on the material design and process conditions of no-flow underfill is discussed.

Metallization of microvias by sputter-deposition

To avoid material stresses in fan-out wafer-level and panel-level packages that create warpage, cracking, delamination, and thermal instabilities, the use of CTE matching and the lowest possible process temperatures has been a focus with this technology. Data is provided to show that this approach is actually causing more stress and crack propagation than is necessary especially for large wafers and panels. An understanding of the source of these chemical, mechanical, and thermal instabilities is offered with methods for avoiding these issues. Reduced warpage and bow, increased fracture energies, and wide-area film uniformity data will be shown to be possible on the largest of substrates with the use of low temperature curing by variable frequency microwave curing. A method is also described for increasing the speed and reducing the cure temperature even further with customized resins. Key words: WLP, PLP, failure, polymer, layers, temperature, stress, warpage, microwaves

In this paper, using epoxy binder properties to strengthen solder joints, such as underfill, without additional process is to ensure reliability. Recently highly integrated electronic components of solder joint reliability and price competitiveness have emerged as very important issues. To meet these needs, lead-free solder reflow profiles without changing the epoxy flux/paste must be available. Prior to melting the solder should not be cured epoxy. The solder joint intensification induces the mechanical thermal crack propagation path by stress concentration to the bulk and the reliability increases due to the crack path where the solder joint intensification becomes longer. There is the effect that it is excellent for ensuring price competitiveness because it can secure reliability without the additional process or materials by using the epoxy flux/paste using epoxy binder characteristics.

Voiding is attributed to the flux outgassing within the solder joints when the solder is at molten state. The effect of reflow profile on voiding at microvia for lead-free soldering is strongly dependent on the flux chemistry. In general, wetting is more important than melting outgasing behavior, and can be enhanced by employing a higher melting energy, including both higher peak temperature and longer dwell time. Use of a high soaking energy can help drying out volatiles hence reduce the melting outgasing and result in low voiding, but may also increase oxidation for pastes with poor oxidation resistance and cause a high voiding. Testing oxidation resistance of solder paste beforehand will promise a more accurate selection of soaking energy.

The electronics industry has widely adopted Sn3Ag0.5Cu solder alloys for lead-free reflow soldering applications. One of the challenges with this alloy is the use of the three percent silver which has caused rapid price increases in lead-free solder alloys because of the limited global availability of silver metal. Low silver containing lead-free alloys have been developed to address the cost issue but they have been mainly found to have reduced thermal cycling reliability performance compared with Sn3Ag0.5Cu. Development was done on a new lead-free solder alloy (Sn0.1Ag0.7Cu0.03Co) as a potential replacement for Sn3Ag0.5Cu. Bulk alloy mechanical testing was done for Sn0.1Ag0.7Cu0.03Co, Sn3Ag0.5Cu, Sn0.3Ag0.7Cu and Sn0.7Cu0.05Ni solder alloys. In additional copper erosion tests were done with the low silver cobalt containing alloy showing reduced copper erosion versus Sn3Ag0.5Cu. Boards were assembled with Sn0.1Ag0.7Cu0.03Co, Sn3Ag0.5Cu, Sn0.3Ag0.7Cu and Sn0.7Cu0.05Ni to understand intermetallic compound (IMC) growth rate after board assembly and thermal ageing as well as wetting/ spreading tests on different board surface finishes. The cobalt containing low silver alloy was found to reduce IMC growth rate after assembly and thermal ageing and had good wetting/spreading performance. Solder joint thermal cycling tests showed equivalent or better reliability for the Sn0.1Ag0.7Cu0.03Co alloy compared with Sn3Ag0.5Cu. Reflow profile development was also done with the 227ºC melting point Sn0.1Ag0.7Cu0.03Co alloy to understand peak temperature and time above liquidus guidelines during board assembly. The results of the tests are reported.

The consumer electronics industry has widely adopted tinsilver- copper (SAC) solder alloys for lead-free reflow soldering applications. The automotive electronics and power device industry demand high thermal fatigue resistance as compared to consumer electronics. Though SAC solder alloys fulfill most soldering requirements alternative solders are needed to meet more stringent environmental regulations, requirements for greater mechanical reliability, and more demanding high temperature service environments such as under the-hood in automobiles and in avionics systems. The alternative Pbfree solder alloy must satisfy both, the process as well as the reliability requirements. The new solders must form joints with acceptable strength, and, at the same time, must withstand thermal fatigue over the projected operational life of the assembly. Mechanical properties and microstructure are, therefore, among the main parameters to be controlled in the candidate solders in an industrial process. Development was done on alternative lead-free alloys with varying percentages of tin-bismuth-indium-silver in comparison with a number of conventional lead-free alloys in soldering evaluations. By addition of small amounts of Indium, tin base solder alloys are strengthened without lowering ductility and improves the thermal fatigue resistance. However, adding large amounts of In causes phase transition from β-phase to γ-phase. β-phase consists of a tetragonal crystal structure with many slip planes and directions and it is therefore very easy to deform. On the other hand, γ-phase consists of a hexagonal crystal structure which consists of lesser number of slip planes and directions than β –phase. Therefore, choosing an accurate amount of Indium to maintain a hexagonal crystal lattice was paramount for this study. As-reflowed and aged samples (1000 hrs. at 150°C in air) were prepared for mechanical and reliability tests. Tests included, tensile tests of solder alloy with micro dumbbell using as cast and after aging, x-ray diffraction was performed on solder powder, cross sections were prepared and microstructure was examined using scanning electron microscopy and energy-dispersive x-ray diffraction. Accelerated thermal cycling test was performed on the boards assembled with various different alternative alloys and the results were compared with conventional Pb-free alloys. It was seen that indium and bismuth containing alloys lasted as much as 40% more than the conventional SAC alloys. The results of the tests are reported.

S.359-366

The paper presents a novel approach on an emerging problem in lead-free reflow soldering technology. The trend of miniaturized SMD device application and the increasing component density cause newfound problems during the reflow process. In this work, the issue of Printed Circuit Board (PCB) shrinkage is inspected in the environment of automotive electronics production. The shrinkage effect results in linear offset on the double sided PCB along the (XY) dimensions during stencil printing of the second reflow pass. The observed phenomenon causes tombstone and bridging failures on specific fine-pitch SMD components. To investigate and obtain deeper understanding of the phenomenon, a new method was developed for the measurement of shrinkage. The novel measurement method is evaluated with a less-productive, but more precise measurement device. With the collected data it is possible to approximate the overall shrinkage of the given product. After introducing a compensation step on the stencil design (based on the measurements), it is possible to significantly reduce the quantity of failures caused by the shrinkage.